Heatsink for LM3886 Guitar Amp

[exztinct01]

Is this enough for a mono LM3886 (two for stereo) guitar power amp?
http://ph.rs-online.com/web/p/products/1898274/
I'm planning to build a stereo amp using two LM3886 now that I'm finished with my LM1875 one. If placing the heatsink fins outside amp chassis would help for better dissipation, then I'm willing to do it with that heatsink.

If not, how 'bout one of these for stereo?
http://ph.rs-online.com/web/p/products/5074876/

http://ph.rs-online.com/web/p/products/1898684/
I know they show their thermal resistances were given but I still need your advice.

[J M Fahey]

All of them will work very well for what you want, taking care of a couple details:
they should be mounted with fins vertical
* if inside, cut a heatsink sized slot on the cabinet above and another below so cold air can flow through
* if outside free air flow is guaranteed.

I use very similar ones, but attach them to backpanel which is made out of aluminum, and transistors are inside but attached to them.

[exztinct01]

thanks Juan. I was hesitant to use the sink in the first link because almost all sinks I saw being recommended for Lm3886 mono is about 200mm long, 100mm wide and 10mm thick. That first one i linked is only 100x100x40 with 10mm thick so I thought it's not enough.

[phatt]

As Juan said,, :tu: 
If you are going to run it hard then bigger will be better.
Just watch out if you are going to run a 4 Ohm load at full bore,, you may need to keep supply rails under ~30VDC if you want it to last.

Data sheet here;
http://www.ti.com/product/lm3886

Info and pics here;
http://www.sound.westhost.com/project19.htm

Gain clone stuff here;
http://diyaudioprojects.com/Chip/LM3886_CA/LM3886_CA.htm

I've actually got a couple of LM3886 PCB's up on the bench right now,, rails are 32VDC.
I would take some pics for you but my Camera has died :grr

I also have a Behringer BiAmp powered monitor came in dead last week which has 3 x LM3886's,, running 42Volt rails :duh
Two are bridged for 4 Ohm LF and one for HF tweeter. The horn has burnt out and that chip latched up to positive rail,  chip is burn finger hot.  Sadly all surface mount double sided board so darn near impossible for me to fix. xP
Phil.

[exztinct01]

when you say "running an amp hard" what does it truly mean?

and is a 160 VA 25-0-25 AC transformer enough for stereo LM3886? Unfortunately, Rod Elliott's P19 does not include details about power supply.

[phatt]

"Drive hard/Run hard" simply means FULL VOLUME flat out (or close to it) which will test the capability of what you build.

With a 4 Ohm load that becomes even harder, ~twice the heat will then need to be dissipated from that die,, Which is why it's mentioned in the data sheets. and I notice a lot of kit instructions also make note of the limits of these chips.

The rule of thumb seems to be; Don't use Max supply rails if you are going to drive 4 Ohm loads.
My LM3886 Jaycar kit is running from 32VDC rails and at full load that will drop a couple of volts so it's likely going to survive a 4 Ohm load but I'm yet to test that out. Too many jobs lined up at the moment.

Read section on "power supply" here for a clue;
http://kitsrus.com/pdf/k106.pdf   
So Yes looks like 160VA should work fine. :tu:

Regarding the 4 Ohm issue;
From the Ti data sheet
LM3886
68W Cont. Avg. Output Power into 4Ω at VCC= +/-28V

38W Cont. Avg. Output Power into 8Ω at VCC= +/-28V

50W Cont. Avg. Output Power into 8Ω at VCC= +/-35V

So yeah 68Watts at 4 Ohms,,,, but likely asking for trouble if driving 4 Ohm at higher supply voltage. xP

Further down that data sheet it mentions "Wide supply range" 20 too 94 Volts. :loco
So go figure? Who really knows how far it can be pushed. :crazy2:

I wish my camera would work as I could show you the setup inside the Behringer powered speaker I mentioned.
No finned heat sink just a thick blob of alloy angle then bolted to an alloy cover plate which aids in heat dissipation. Not 1 but 3 LM3886 chips !!!
2 are bridged driving the 6 inch 4 Ohm woofer (Yikes!!) and running from +/-42VDC ARRRG Scary.
To it's credit it lasted about 8 Years before it finally died.

Phil.

[exztinct01]

I don't know why I'm always worried that it won't work unless I follow the recommended parts
I don't even feel satisfied unless I build it with overkill parts :loco

I'll try to build it then using parts I can afford, then try to upgrade if necessary.
Thanks Phil.



~ Stephen

[phatt]

Hey Stephen,, never fear you are do'in just fine,, it pays to ask if you are not sure.
Yep, No need to redesign the space shuttle just yet. :lmao:

Hey I just realized that the memory card in the camera still has some pictures and silly me just realized that they can be read directly without the camera. :duh

So here is the guts of that powered speaker.
The transformer reads 27.8 -0- 27.8 VAC @ 139VA (2.5Amps)
which should deliver 34 -0- 34 VDC @ 2.2Amps.
That was running 3x LM3886 so I doubt you would need more.

My meter reads over 40VDC so maybe the trafo was wound for 220Volt primary? (not uncommon)

Note the pic,, obviously very little effort went into design of the heat sink. The whole unit is just screwed to the back of the speaker box.

A Safety point worth a comment;
Notice how the Primary wires are next to the Secondary wires?
Well they were twisted together and zip tied.

Q? What would happen if they melted?  8|

OK tiss unlikely but still possible in a major failure.
You then have Mains potential on the circuit,, meanwhile the singer holding the microphone just died.

If this was an electrical device in a Hospital it would be classed as deadly dangerous layout and banned never to be used. The manufacturer would likely get a nasty letter and get no return work.

So keep it in mind when you build stuff.  :tu:
Phil.

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[J M Fahey]

I don't know why I'm always worried that it won't work unless I follow the recommended parts

Cool, but it depends a lot on *who* recommended them

It´s in the datasheet? (as were the voltages and impedances mentioned by phatt) .... cool, who knows a chip better than its designers and makers ?

Read "somewhere in the Net"? , written by an unknown who maybe didn`t even ever actually *build* the thing but feels forced to write "something"?
Take it with a grain of salt.

Typically they will try to err on the safe side and overspec ... so far it´s not bad.

But then somebody else who didn´t build it either also wants to write "something" , has a vague memory of what he read somewhere else and "just in case" again overspecs.

Add 3 or 4 of those cascaded overspec`ings and you end up using this to adjust some nuts in your bike:

[exztinct01]

First - Have a good power supply circuit and a big enough/proper voltage transformer
Second - Big heatsink, there is no such thing as too large of a heatsink

Proper heatsink size for two LM1875 chips, or one LM3886:

He said it. Proper size for the LM3886 accdg to Joe